Electrochemical photovoltaic cells (EPC's) are based on a junction between a semiconductor (p-type or n-type) and an electrolyte containing one redox couple; an auxiliary electrode completes the device. If the semiconductor and electrolyte Fermi levels are different and well suited, a built-in potential will develop at their interface and the device will exhibit diode rectification in the dark. When electrons and holes are photogenerated in the vicinity of the junction, the built-in potential permits separation of the charges. If a n-type material is used, holes (valence band) will migrate to the interface and allow oxidation of reduced species contained in the electrolyte. At the same time, photogenerated electrons (conduction band) will migrate toward the bulk of the semiconductor to reach the auxiliary electrode, via an external circuit, where they will reduce the oxidized species of the electrolyte. If a p-type material is used, the processes are reversed: photoelectrochemical reduction at the semiconductor/electrolyte interface, and electrochemical oxidation at the auxiliary electrode/electrolyte interface. As the reactions involve the same redox couple, there is no net chemical change in the electrolyte (ΔG=0) and therefore the effect of the device illumination is to produce a photocurrent and a photovoltage (photovoltaic effect). Such devices can serve as photodiodes (monochromatic light) and as solar cells (white light). The maximum open-circuit photopotential (Voc) is determined by the difference between the Fermi level of the semiconductor and that of the electrolyte, the latter being fixed by the redox potential.
EPC's are very attractive for the production of electricity and present a number of advantages over the p-n heterojunctions. The latter generally need a doping step and interdiffusion of majoritary carriers between the p and n regions, whereas the semiconductor/electrolyte junction is simply formed by transfer of majoritary carriers from the semiconductor to the electrolyte, on immersion of the semiconductor into the electrolyte. Among other advantages we can stress: (i) the elimination of light energy losses by absorption in one half of the junction if the electrolyte is colorless; (ii) the possibility of using a thin film polycrystalline semiconductor (much less expensive than a single crystal) with only a small decrease in the cell energy conversion efficiency; (iii) the large number of redox couples (and thus of electrolyte Fermi levels) that can be used to vary the junction built-in potential and hence the device photopotential and photocurrent.
There is extensive prior art on EPC's. The direct conversion of solar energy to electricity by using a semiconductor/electrolyte interface has been demonstrated by H. Gerischer and J. Goberecht in Ber. Bunsenges Phys. Chem., 80, 327 (1976), and by Ellis et al. in J. Am. Chem. Soc., 98, 1635 (1976). The Gerischer cell consisted of a n-CdSe single crystal photoanode and of a doped SnO2 conducting glass cathode dipped in an aqueous alkaline electrolyte containing the Fe(CN)64−/Fe(CN)63− redox couple. The energy conversion efficiency was 5% but the cell performance decreased rapidly due to decomposition of the illuminated semiconductor electrode. Since that time a major effort has been devoted to the technology of solar energy conversion and to fabrication of various single crystal and polycrystalline semiconductors such as CdS, CdSe, CdTe, WS2, WSe2, MoS2, MoSe2, GaAs, CuInS2, CuInSe2 and CuIn1-xGaxSe2. Most of the cells used an aqueous electrolyte (various redox couples were studied: Fe(CN)64−/Fe(CN)63−, I−/I3−, Fe2+/Fe3+, S2−/Sn2−, Se2−/Sen2−, V2+/V3+) and systems exhibiting a good energy conversion efficiency were generally unstable under sustained illumination due to a process called photocorrosion. The use of a solvent-free polymer electrolyte could eliminate the photocorrosion process owing to its larger electrochemical stability window and to the low solvation energy for the ions that compose the semiconductor materials. Furthermore, this medium allows the fabrication of compact devices with no leakage of solvent, giving a lower absorption of visible light by the electrolyte. Few EPC's based on the junction between protected n-Si single crystal and poly(ethylene oxide), PEO, complexed with a mixture of KI and I2, were investigated but their stability has not been demonstrated (T. A. Skotheim, Appl. Phys. Lett., 38, 712 (1981), T. A. Skotheim et al. Journal de Physique, C3, 615 (1983), T. A. Skotheim and O. Inganäs. J. Electrochem. Soc., 132, 2116 (1985)).
A. K. Vijh and B. Marsan in Bull. Electrochem., 5, 456 (1989) have demonstrated that the all-solid-state EPC's n-CdSe (polycrystalline)∥high molecular weight PEO-based copolymer (noted as modified PEO) complexed with M2S/xS (M=Li, Na, K; x=1, 3, 5, 7)∥indium tin oxide conducting glass (ITO) are very stable under white light illumination. However, these authors showed that the high series resistance of the cells, mainly attributed to the very low ionic conductivity of the polymer electrolytes, control the device performance.
In order to enhance the conductivity of the solid electrolyte, a cesium thiolate (CsT)/disulfide (T2) redox couple, where T− stands for 5-mercapto-1-methyltetrazolate ion and T2 for the corresponding disulfide, was dissolved in modified PEO and studied in an EPC (J. -M. Philias and B. Marsan, Electrochim. Acta, 44, 2915 (1999)). It was found that the PEO12−CsT/0.1 T2 electrolyte composition, which is transparent to visible light, exhibits the highest ionic conductivity with 2.5×10−5 S cm−1 at 25° C. (J. -M. Philias and B. Marsan, Electrochim. Acta, 44, 2351 (1999)). Under white light illumination, the cell possesses an energy conversion efficiency (0.11% at 50° C.) about 5 times higher than that of the previous configuration. The lower cell series resistance and the more anodic potential of the T−/T2 redox couple (0.52 V vs NHE as compared to —0.34 V for the Sn2−/Sn+12− couple) are largely responsible for this improvement. When the EPC is illuminated, thiolate ions (T−) are photooxidized at the n-type semiconductor electrode (forming the S—S bond of the T2 disulfide species) whereas T2 species are reduced at the conducting glass electrode (with cleavage of the S—S bond). Despite this improvement, the conductivity of the solid polymer electrolyte is still too low, particularly at room temperature, and continues to limit the cell performance. EPC's incorporating a much higher conductive gel electrolyte (˜10−3 S cm−1 at 25° C.) were reported in the literature, for example by Cao et al. in J. Phys. Chem., 99, 17071 (1995), and Mao et al. in J. Electrochem. Soc., 145, 121 (1998). This type of electrolyte consists in the introduction of an aprotic liquid electrolyte in a polymeric matrix. The polymer gives good mechanical properties whereas the liquid electrolyte is responsible for the good conductivity and electrode wetting. Renard et al. in Electrochim. Acta, 48/7, 831 (2003) found that the dissolution of the T−/T2 redox couple in a mixture of DMF and DMSO, and incorporated in poly(vinylidene fluoride), PVdF, gives transparent and highly conductive gel electrolytes (conductivities up to 7×10−3 S cm−1 at 25° C.) with very good mechanical properties. However, when this electrolyte replaced the solid ionic membrane PEO12−CsT/0.1 T2 in an EPC, the cell conversion efficiency was not improved.
It has been demonstrated that the cell performance is actually limited by the very slow reduction kinetics of the oxidized species (T2) at the transparent ITO auxiliary electrode and that the difference between oxidation potential of T− and reduction potential of T2 at this electrode is as large as 3.06 V in a PVdF-based gel electrolyte containing 50 mM CsT and 5 mM T2. Other authors previously reported low cathodic charge transfer between ITO and aqueous polysulfide (S2−,S, OH−) (Tenne et al., Ber. Bunsenges Phys. Chem., 92, 42 (1988)) or polyiodide (I−, I2) solutions (Tenne et al., J. Electroanal. Chem., 269, 389 (1989)).
Hodes et al. in J. Electrochem. Soc., 127, 544 (1980) found that the transition metallic sulfides Cu2S and CoSx act as good electrocatalysts for the polysulfide redox reactions. However, the former is mechanically instable in the electrolyte.
U.S. Pat. No. 4,421,835 describes that cobalt sulphide can be deposited on a conducting substrate such as brass. Such a deposition is carried out by first depositing hydrous cobalt hydroxide and then by converting the latter into cobalt sulphide by treating it with a sulphide solution. However, this document does not teach nor suggest how to deposit cobalt sulphide on a non-conducting substrate.
U.S. Pat. No. 4,828,942 describes a thin cobalt sulphide electrode which can be produced by electrodeposition of cobalt onto a brass foil followed by alternating anodic and cathodic treatment in polysulfide solution. However, this document does not teach nor suggest how to deposit cobalt sulphide on a non-conducting substrate.
U.S. Pat. No. 5,648,183 describes an electrocatalytic electrode comprising a porous material such as cobalt sulphide deposited on a porous nickel or porous brass. However, this document does not teach or suggest how to deposit cobalt sulphide on a non-conducting substrate.
It has been shown that deposited Co(II) species can serve as an electrocatalyst for the reduction of Sn2− ions on ITO electrode (Tenne et al., Ber. Bunsenges Phys. Chem., 92, 42 (1988)) or p-InP photoelectrode (Liu et al., J. Electrochem. Soc., 129, 1387 (1982)).
A method of depositing cobalt sulfide on ITO has been reported by Tenne et al. in Ber. Bunsenges Phys. Chem., 92, 42 (1988). The latter method consists in immersing the substrate for a few minutes in CoCl2 solution (≧0.1 M), rinsing in water and then immersing in a separate polysulfide solution for a few minutes; this process can be repeated several times. However, this technique does not allow an adequate control of the COS film thickness.
Hodes et al. in J. Electrochem. Soc., 127, 544 (1980) reported the preparation of a CoS thin film on stainless steel. The two-steps method involves the electrodeposition, at 25° C. and for few minutes, of Co(OH)2 onto the metallic substrate, from an aqueous solution of CoSO4 with a potassium biphthalate buffer, at a current density that depends on the pH of the electrolyte. When immersed in a polysulfide solution, Co(OH)2 is converted to cobalt sulfide, mainly CoS. However, when the above method is used to form a cobalt sulfide layer on a transparent conducting glass electrode (ITO), metallic cobalt is plated on the substrate (instead of Co(OH)2) during the first step, that cannot be converted to COS by a subsequent immersion in the polysulfide solution.
Thus, it seems to be very difficult to fabricate, on ITO, COS thin films of easily controllable thicknesses (and therefore transparencies).